The mayoral choice has boiled down to the polarized essence of Democrat (Filner) vs. Republican (DeMaio).

In Oceanside, on the other hand, the three-man mayoral race is interesting precisely because party politics, thanks to a wild card, have become really complicated.

To start, Terry Johnson, a truly historic figure (the first black mayor in the county’s history), has no obvious business running for mayor other than altering the partisan dynamic of the race.

After a dozen years on the council, lifelong Democrat Johnson was defeated in 2004 for re-election as a newly minted pro-business Republican mayor, undone by a three-candidate race that included the popular current mayor, Jim Wood.

In retrospect, Johnson’s chances were spoiled by another pro-business Republican, then-Councilman Rocky Chavez.

In quick succession, Johnson then lost a special election for Wood’s open council seat, a stunning — and seemingly career-ending — defeat. Two quick strikes and Johnson was yesterday’s news.

Clearly, Johnson’s political conversion, orchestrated by the late political consultant Jack Orr, didn’t work out.Johnson changed his spots — and suffered at the ballot box.

After eight years out of public office, Johnson’s back as a Democrat. He says he changed his registration to vote for Obama in 2008.

The real Oceanside mayoral race, most believe, is between Republican Wood and Councilman Jerry Kern, a GOP Central Committee member whom the public unions and neighborhood activists tried and failed to recall in a bitter election a few years ago.

More recently, Kern pushed a phaseout of mobile-home rent control, which was defeated at the ballot box in June, hardening Oceanside’s partisan lines.

In Oceanside, the conventional wisdom goes, candidate Kern will have a hard time cracking 50 percent in a general election.

Though a Republican, Wood is widely considered by GOP hard-liners to be an anti-Republican, a RINO, a public union ally, supporter of mobile-home rent control and staunch opponent to outsourcing city services, a cost-saving strategy endorsed by Kern and the local Republican orthodoxy.

Without Johnson in the race, Wood likely cakewalks to a third term, thanks in large part to moderate Republicans, Democrats and independents. Kern has to struggle to get out of the box of GOP ideologue, though he’s a smart and consistent politician who explains his pro-business positions with teacherly patience.

With Johnson on the ballot, however, Wood’s base gets whittled away if — and it’s a big if — Democrats vote according to party. If that happens, enough votes could be sucked away from Wood that Kern could win with a plurality. Johnson could experience the satisfaction of helping defeat the man who beat him in 2004.

I ran this analysis past Jess Durfee, the county Democratic Party leader.

Durfee told me that the Carlsbad-Oceanside Democratic Club did not consider endorsing Johnson and specifically asked the county Democratic Central Committee not to endorse him.